The present invention generally relates to an integrated semiconductor circuit and a method of producing it, said circuit comprising at least two mutually separated wafer parts of semiconductor material, and conductors for establishing electrical connection between the wafer parts which are substantially thermally insulated from each other.
The invention further relates to the use of such an integrated semiconductor circuit for providing a flow meter for measuring the flow velocity of a flowing gaseous or liquid medium, said flow meter being particularly characterized in that it comprises two mutually separated wafer parts one of which is adapted to be heated and disposed in the flow and the other of which is also adapted to be disposed in the flow, however without being heated, the flow velocity of the medium being calculated on the basis of the dissipation by thermal convection of the first wafer part to the ambient flowing medium.
Since the invention is especially well suited for providing flow meters or flow sensors of the above-indicated type, the following description will be directed to the use of the integrated semiconductor circuit and the method of producing it especially for such sensors, but it is evident to anyone skilled in the art that this is not the only field of application of the invention which may also be used in all integrated circuits where it is desirable to achieve thermal insulation between different parts of a semiconductor circuit and electrical connection between the parts.
In semiconductor technology, a large number of integrated transducers or sensors of semiconductor material have been developed lately, such as temperature and flow sensors, which are manufactured along the same principles as conventional integrated circuits, i.e. are built up on a layer of monocrystalline semiconductor material on which the electric components and conductors required for the operation of the sensor have been integrated according to known techniques. In addition to the small size of the sensor and the reduced costs of manufacture of the sensor per se, achieved by producing several identical units at a time (batch process), a further advantage is gained, namely that the signal processing electronic circuits or equivalent components associated with the sensor can be directly integrated on the sensor in connection with the manufacture thereof, which further reduces the cost of the sensor and enhances its reliability. The measuring performance of the entire system is also improved by directly integrating the signal processing electronic circuits on the sensor. A first signal gain can then be achieved closer to the measuring unit proper, which prevents weak signals from disadvantageously being fed over long signal paths.
A known flow velocity sensor of this type comprises a thin, narrow silicon beam, a base plate fixedly connected to one end of the silicon beam and carrying bonding pads required for the operation of the sensor, and a sensor part, also of silicon, fixedly connected to the other end of the silicon beam. For using the sensor, said other end of the beam is inserted through a tube wall or the like into a flow of gas or liquid, the velocity of which should be measured, so as to place the sensor part in the flow. The mode of operation of the sensor, which is based on known techniques, is as follows. The sensor part is electrically heated by means of a resistor integrated thereon, to an upper temperature, whereupon the sensor part is allowed to cool to a lower temperature as a result of the dissipation by thermal convection, it being possible to repeat this process cyclically. Both the heating time and the cooling time are indicative of the flow velocity of the medium. A first temperature-sensitive diode for compensating for temperature variations in the medium is integrated in the silicon beam, and a second temperature-sensitive diode is integrated in the sensor part, it being possible by means of this second diode and the resistor to provide a temperature feedback control system for controlling the temperature of the sensor part.
In order to achieve accurate flow measurements with the flow sensor described above, it is obviously desirable that the sensor part be thermally insulated from the silicon beam, such that the temperature of the sensor part is substantially affected by the dissipation by thermal convection because of the flow and not because of thermal conduction between the sensor part and the beam.
To this end, the circuit in the above-mentioned previously known flow sensor has been formed into two physically separated units or wafer parts which are held together only by the conductors extending between the wafer parts (sensor part and beam) and also providing an electrical connection between the wafer parts. This solution however suffers from a serious drawback. So that the conductors should have a sufficient supporting capacity, i.e. in order that the beam in the above-described flow sensor should be able to support the sensor part, they must have a relatively large thickness, which means a large conductor cross-sectional area in the joint between the sensor part and the beam and, thus, entails undesired thermal conduction by the conductors from the sensor part to the beam, which in turn adversely affects the sensitivity and speed of the sensor device. If the conductors are made thinner to prevent such undesired thermal transfer between the sensor part and the beam, the sensor device will become more vulnerable to impacts and more easily damaged. There is also a risk that a sensor device of the above-defined type will be damaged because of the pressure from the ambient flowing medium.